Multicarrier modulation is widespread for radio communication or wire communication. The multicarrier modulation is a scheme in which transmission data is divided, and the divided transmission data are assigned to a plurality of carriers, respectively, and the carriers corresponding to the respective transmission data are modulated, and the modulated carriers are multiplexed. The multicarrier modulation called OFDM (Orthogonal Frequency Division Multiplexing), which is a type of multicarrier modulation, is widespread for practical use in the fields of digital terrestrial television broadcasting, wireless LAN (Local Area Network), xDSL (Digital Subscriber Line), power line communication (PLC: Power Line Communication), and the like. As specific exemplary applications, DVB-T is used for digital terrestrial television broadcasting, IEEE802.11a is used for wireless LAN, ADSL is used for xDSL, Home Plug is used for power line communication. In the following description, in particular, the multicarrier modulation using the aforementioned typical OFDM is referred to as OFDM/QAM (Quadrature Amplitude Modulation) multicarrier modulation or simply as OFDM/QAM, so as to be distinguishable from other multicarrier modulations.
(As to OFDM/QAM)
The principle of the OFDM/QAM is described in Non-Patent Document 1.
The OFDM/QAM is a multicarrier modulation for individually subjecting a plurality of carries having different frequencies from each other to complex vector modulation at every symbol transmission cycle, and multiplexing the modulated carriers. When Ts represents a time interval of a symbol, a plurality of the symbols being continuous in the time direction, and fs represents a frequency interval between carriers adjacent to each other in the frequency direction, a general expression representing a transmission signal for the OFDM/QAM can be represented by (Equation 1).
                              s          ⁡                      (            t            )                          =                  Re          [                                    ∑              m                        ⁢                                          ∑                k                            ⁢                                                d                                      m                    ,                    k                                                  ⁢                                  g                  ⁡                                      (                                          t                      -                                              mT                        s                                                              )                                                  ⁢                                  ⅇ                                      j2π                    ⁡                                          (                                                                                                    f                            c                                                    ⁢                          t                                                +                                                                              kf                            s                                                    ⁡                                                      (                                                          t                              -                                                              mT                                s                                                                                      )                                                                                              )                                                                                                    ]                                    [                  Equation          ⁢                                          ⁢          1                ]            
In (Equation 1), m represents a symbol number, k represents a carrier number, fc represents a reference frequency of a carrier, and t represents a time. dm,k represents a complex vector indicating transmission data to be transmitted on the k-th carrier in the m-th symbol. g(t−mTs) represents a window function for the m-th symbol, which is obtained by time-shifting a window function g(t) to the m-th symbol. For example, the window function g(t) is defined as (Equation 2).
                              g          ⁡                      (            t            )                          =                  {                                                                      1                  ,                                                                                                  -                                          T                      g                                                        <                  t                  ≤                                      T                    u                                                                                                                        0                  ,                                                            otherwise                                                                        [                  Equation          ⁢                                          ⁢          2                ]            
One symbol interval include a guard interval and an effective symbol interval. In (Equation 2), Tg represents a guard interval length, Tu represents an effective symbol interval length, and a relationship between Tg and Tu satisfies Ts=Tg+Tu. Further, a relationship between the effective symbol interval length Tu and a carrier frequency interval fs satisfies Tu=1/fs.
In the OFDM/QAM, a signal corresponding to the effective symbol interval length Tu is extracted from the symbol interval Ts including the guard interval Tg and demodulated in receiving process performed at the reception end.
In a wireless mobile communication, a multipath propagation often causes a problem. The multipath propagation represents a phenomenon that a plurality of transmission signals arriving at various times in delay due to reflection of a radio wave are multiplexed and received at the reception end. In the OFDM/QAM, the symbol includes a guard interval such that an arrival time difference, caused due to the multipath propagation, among a plurality of incoming waves is absorbed so as to maintain the orthogonal characteristic among a plurality of carriers. The guard interval enables a signal to be received at the reception end even in a multipath propagation environment without causing an interference between symbols and an interference between carriers. Such a resistance to the multipath propagation is utilized in the field of a radio communication such as digital terrestrial television broadcasting and wireless LAN.
On the other hand, in the wire communication such as xDSL and power line communication, an interfering signal received from another system and/or device often causes a problem. In many cases, such an interfering signal is a narrowband signal. In the OFDM/QAM, modulated waves transmitted on the respective carriers are received by narrow-passband filters, respectively. Therefore, in the OFDM/QAM, a carrier influenced by the interfering signal can be limited. Further, in the OFDM/QAM, an amount of information of data transmitted on a carrier influenced by the interfering signal is reduced, or the carrier is not used, thereby enabling improvement of the resistance to the interfering signal. In the field of the wire communication such as the xDSL and power line communication, such a resistance to the narrowband interfering signal is utilized.
However, the guard interval for the OFDM/QAM is used only for absorbing the influence of the multipath, and is not used for transmitting effective information. Therefore, as described in Patent Document 1, the OFDM/QAM has a problem that an efficiency with which a spectrum is used is low, and an energy loss is large.
The OFDM/QAM has another problem that, when a delay time caused by the multipath propagation is longer than the guard interval of an incoming wave, transmission quality is abruptly deteriorated. The OFDM/QAM has still another problem that a passband of a filter for receiving a modulated wave is not sufficiently narrow, and an effect of limiting a carrier influenced by a narrowband interfering signal is insufficient.
As described in Patent Document 1, an OFDM/OQAM (Offset Quadrature Amplitude Modulation) multicarrier modulation is known as another multicarrier modulation for solving the aforementioned problems. In the following description, the OFDM/OQAM multicarrier modulation is simply referred to as an OFDM/OQAM.
(As to OFDM/OQAM)
The principle of the OFDM/OQAM is described in Patent Document 1, and Non-Patent Document 2.
The OFDM/OQAM is a multicarrier modulation for individually subjecting a plurality of carries having different frequencies from each other to amplitude modulation at every symbol transmission cycle, and multiplexing the modulated carriers. When Ts represents a time interval of a symbol, a plurality of the symbols being continuous in the time direction, and fs represents a frequency interval between carriers adjacent to each other in the frequency direction, a general expression representing a transmission signal for the OFDM/OQAM can be represented by (Equation 3).
                              s          ⁡                      (            t            )                          =                  Re          [                                    ∑              m                        ⁢                                          ∑                k                            ⁢                                                d                                      m                    ,                    k                                                  ⁢                                  g                  ⁡                                      (                                          t                      -                                              mT                        s                                                              )                                                  ⁢                                  ⅇ                                                                                    j2π                        ⁡                                                  (                                                                                    f                              c                                                        +                                                          kf                              s                                                                                )                                                                    ⁢                      t                                        +                                          jϕ                                              m                        ,                        k                                                                                                                          ]                                    [                  Equation          ⁢                                          ⁢          3                ]            
In (Equation 3), m represents a symbol number, k represents a carrier number, fc represents a reference frequency of a carrier, and t represents a time. dm,k represents an amplitude value indicating transmission data transmitted on the k-th carrier in the m-th symbol. g(t−mTs) represents a window function for the m-th symbol, which is obtained by time-shifting a window function g(t) to the m-th symbol. Φm,k represents a modulation phase represented by (Equation 4). In the OFDM/OQAM, the modulation is performed such that a phase is different by π/2 radians between symbols adjacent to each other in the time direction, and between carriers adjacent to each other in the frequency direction.
                              ϕ                      m            ,            k                          =                  {                                                                                          π                    /                    2                                    ,                                                                                                  (                                          m                      +                      k                                        )                                    ⁢                                                                          ⁢                  odd                                                                                                      0                  ,                                                                                                  (                                          m                      +                      k                                        )                                    ⁢                                                                          ⁢                  even                                                                                        [                  Equation          ⁢                                          ⁢          4                ]            
In the OFDM/OQAM, a relationship between the time interval Ts of each of the plurality of the symbols which are continuous in the time direction and the frequency interval fs between carries adjacent to each other in the frequency direction satisfies Ts=1/(2fs).
Accordingly, when the carrier frequency interval fs is set so as to be the same between in the OFDM/OQAM and in the OFDM/QAM for making a comparison, it is notable that, in the OFDM/OQAM, phase axes orthogonal to each other at half (but not exactly half since a symbol includes a guard interval in the OFDM/QAM) the symbol transmission cycle for the OFDM/QAM are alternately subjected to the amplitude modulation. Anything which has the characteristic of the OFDM/OQAM is handed as the OFDM/OQAM, and the present invention is applied thereto. For example, the time-limited orthogonal multicarrier modulation described in Non-Patent Document 3, the DWMT (Discrete Wavelet Multitone) described in Non-Patent Document 4, and the OFDM/MSK and the OFDM/IOTA described in Patent Document 1 are all regarded as being the same in type as OFDM/OQAM, and are generically referred to as the OFDM/OQAM in the following description.
On the other hand, in a system to which the OFDM/QAM is applied, used is a frame format including a reference signal called a pilot signal which is known to both the transmission and the reception end, so as to estimate transmission characteristic of a transmission channel, and a frequency error and/or a phase error between the transmission end and the reception end.
For example, DVB-T (ETS300-744) which is a standard of digital terrestrial television broadcasting in Europe uses a frame format as shown in FIG. 9. FIG. 9 shows a clipped part of a frame format represented on a time-frequency coordinate plane. In FIG. 9, the abscissa axis represents symbols located in the time direction, and the ordinate axis represents carriers located in the frequency direction. A number on the abscissa axis represents a symbol number in the time direction whereas a number on the ordinate axis represents a carrier number in the frequency direction. A mark ◯ represents a pilot signal called a scattered pilot in the DVB-T, and a mark X represents a data transmission signal. As shown in FIG. 9, the pilot signal is located every 12 carriers in the frequency direction. Further, the pilot signal is located so as to be shifted three carriers in the frequency direction for every symbol proceeding in the time direction. Since the pilot signal is shifted three carriers for every symbol, one cycle of a location pattern of the pilot signals is a four-symbol period. The pilot signal is a signal obtained through modulation performed by using a modulation vector known to both the transmission end and the reception end, and is a signal obtained through modulation performed by using a modulation vector which is predetermined in accordance with the number of the located carrier. A transmission signal transmitted from the transmission end is received as a reception signal at the reception end through a transmission channel. The reception signal received at the reception end has an amplitude and a phase which are different from those of the transmission signal transmitted from the transmission end, depending on the transmission characteristic of the transmission channel. When the reception end observes that the pilot signal is included in the reception signal, the reception end can estimate the transmission characteristic of the transmission channel. Further, the transmission characteristic of the transmission channel, which is estimated based on the pilot signal, is two-dimensionally interpolated in the time direction and the frequency direction so as to estimate the transmission characteristic of the transmission channel for the data transmission signal. The reception end is allowed to correctly demodulate the transmitted data by correcting an amplitude and a phase of the reception signal based on the estimated transmission characteristic of the transmission channel.
In another example, a communication frame of IEEE802.11a which is a standard of a wireless LAN uses a frame format illustrated in FIG. 10. FIG. 10 shows one packet frame represented on a time-frequency coordinate plane. In FIG. 10, the abscissa axis represents symbols located in the time direction, and the ordinate axis represents carriers located in the frequency direction. A number on the abscissa axis represents a symbol number in the time direction whereas a number on the ordinate axis represents a carrier number in the frequency direction. A mark ∘ represents a reference signal including a pilot signal, and a mark X represents a transmission parameter signal or a data transmission signal for transmitting data. The reference signals each including the pilot signal are modulated by using modulation vectors, respectively, known to the reception end. In FIG. 10, signals transmitted in the first symbol and the second symbol, respectively, are called short training sequence, and are used mainly for automatic gain control (AGC), automatic frequency control (AFC), and detection of packets, at the reception end. Signals transmitted in the third symbol and the fourth symbol, respectively, are called long training sequence, and are used mainly for symbol synchronization, and estimation of the transmission characteristic of the transmission channel at the reception end. The fifth symbol is used mainly for transmitting transmission parameter information called SIGNAL. The sixth to the m-th symbols are symbols used mainly for transmitting data. The pilot signals included in the fifth to the m-th symbols, respectively, are used mainly for estimating, at the reception end, phase shifting caused by carrier frequency shifting and sampling frequency shifting from those of the transmission end. The reception end is allowed to correctly demodulate transmitted data by correcting the received signal based on the estimated transmission characteristic of the transmission channel and phase shifting.
However, as described in Patent Document 2, it is difficult to apply, to the OFDM/OQAM, a frame format similar to that used for the OFDM/QAM, that is, a frame format in which the pilot signals are located. The reason will be described with reference to FIG. 11.
FIG. 11 is a diagram illustrating a part of a frame format represented on a time-frequency coordinate plane. In FIG. 11, the abscissa axis represents symbols located in the time direction, and the ordinate axis represents carriers located in the frequency direction. A number on the abscissa axis represents a symbol number in the time direction where as a number on the ordinate axis represents a carrier number in the frequency direction. A pilot signal 10 is located on the k-th carrier in the m-th symbol and transmitted. Data transmission signals 11 to 18 are located in the vicinity of the pilot signal 10 and transmitted. The data transmission signals 11 to 13 are located on the (k−1)th to the (k+1)th carriers, respectively, in the (m−1)th symbol, and transmitted. The data transmission signals 14 and 15 are located on the (k−1)th carrier and (k+1)th carrier, respectively, in the m-th symbol, and transmitted. The data transmission signals 16 to 18 are located on the (k−1)th to the (k+1)th carriers, respectively, in the (m+1)th symbol, and transmitted.
The pilot signal 10 is obtained through modulation performed by using an amplitude value dm,k known to the transmission end and the reception end. The data transmission signals 11 to 18 are obtained through modulation performed by using amplitude values dm−1,k−1, dm−1,k, dm−1,k+1, dm,k−1, dm,k+1, dm+1,k−1, dm+1,k, and dm+1,k+1, respectively, based on the transmission data.
The pilot signal 10 received at the reception end in an ideal state is represented by (Equation 5). The “ideal state” described above represents a state where an amplitude change and a phase shifting do not occur through transmission, or disturbance such as a noise and an interference does not occur, and a transmission signal transmitted from the transmission end is received as it is as a reception signal at the reception end.
                              r                      m            ,            k                          =                              d                          m              ,              k                                +                      j            ⁢                          {                                                                                                                                            α                                                                                    m                              -                              1                                                        ,                                                          k                              -                              1                                                                                                      ⁢                                                  d                                                                                    m                              -                              1                                                        ,                                                          k                              -                              1                                                                                                                          +                                                                        α                                                                                    m                              -                              1                                                        ,                            k                                                                          ⁢                                                  d                                                                                    m                              -                              1                                                        ,                            k                                                                                              +                                                                                                                                                                                    α                                                                                    m                              -                              1                                                        ,                                                          k                              +                              1                                                                                                      ⁢                                                  d                                                                                    m                              -                              1                                                        ,                                                          k                              +                              1                                                                                                                          +                                                                        α                                                      m                            ,                                                          k                              -                              1                                                                                                      ⁢                                                  d                                                      m                            ,                                                          k                              -                              1                                                                                                                          +                                                                                                                                                                                    α                                                      m                            ,                                                          k                              +                              1                                                                                                      ⁢                                                  d                                                      m                            ,                                                          k                              +                              1                                                                                                                          +                                                                        α                                                                                    m                              +                              1                                                        ,                                                          k                              -                              1                                                                                                      ⁢                                                  d                                                                                    m                              +                              1                                                        ,                                                          k                              -                              1                                                                                                                          +                                                                                                                                                                                    α                                                                                    m                              +                              1                                                        ,                            k                                                                          ⁢                                                  d                                                                                    m                              +                              1                                                        ,                            k                                                                                              +                                                                        α                                                                                    m                              +                              1                                                        ,                                                          k                              +                              1                                                                                                      ⁢                                                  d                                                                                    m                              +                              1                                                        ,                                                          k                              +                              1                                                                                                                                                                                      }                                                          [                  Equation          ⁢                                          ⁢          5                ]            
In (Equation 5), rm,k represents a complex vector representing the pilot signal 10 received in the ideal state. αm−1,k−1, αm−1,k, αm−1,k+1, αam,k+1, αm,k+1, αm+1,k−1, αm+1,k, and αm+1,k+1 represent coefficients of specific interferences, to the pilot signal 10, caused by the data transmission signals 11 to 18, respectively. As represented by (Equation 5), the pilot signal 10 {rm,k} received in the ideal state at the reception end includes, in the real number term (in-phase), an amplitude value dm,k of the pilot signal 10 obtained through modulation at the transmission end, and includes, in the imaginary number term (quadrature phase), the specific interferences caused by the data transmission signals 11 to 18 which are located in the vicinity of the pilot signal 10, and transmitted.
Next, communication performed through a transmission channel will be described. When Hm,k represents transmission characteristic of the transmission channel for the k-th carrier in the m-th symbol corresponding to the pilot signal 10, the pilot signal 10 {r′m,k} received through the transmission channel at the reception end is represented by (Equation 6). Here, the transmission characteristic Hm,k is represented as a complex vector.r′m,k=Hm,krm,k  [Equation 6]
In order to estimate the transmission characteristic Hm,k of the transmission channel by using a pilot signal r′m,k received through the transmission channel at the reception end, the pilot signal r′m,k received through the transmission channel may be divided by the pilot signal rm,k to be received in the ideal state, as represented by (Equation 7).
                                                                        H                                  m                  ,                  k                                            =                            ⁢                                                r                                      m                    ,                    k                                    ′                                /                                  r                                      m                    ,                    k                                                                                                                          =                            ⁢                                                r                                      m                    ,                    k                                    ′                                                                      d                                          m                      ,                      k                                                        +                                      j                    ⁢                                          {                                                                                                                                                                                                    α                                                                                                            m                                      -                                      1                                                                        ,                                                                          k                                      -                                      1                                                                                                                                      ⁢                                                                  d                                                                                                            m                                      -                                      1                                                                        ,                                                                          k                                      -                                      1                                                                                                                                                                  +                                                                                                α                                                                                                            m                                      -                                      1                                                                        ,                                    k                                                                                                  ⁢                                                                  d                                                                                                            m                                      -                                      1                                                                        ,                                    k                                                                                                                              +                                                                                                                                                                                                                                                            α                                                                                                            m                                      -                                      1                                                                        ,                                                                          k                                      +                                      1                                                                                                                                      ⁢                                                                  d                                                                                                            m                                      -                                      1                                                                        ,                                                                          k                                      +                                      1                                                                                                                                                                  +                                                                                                α                                                                      m                                    ,                                                                          k                                      -                                      1                                                                                                                                      ⁢                                                                  d                                                                      m                                    ,                                                                          k                                      -                                      1                                                                                                                                                                  +                                                                                                                                                                                                                                                            α                                                                      m                                    ,                                                                          k                                      +                                      1                                                                                                                                      ⁢                                                                  d                                                                      m                                    ,                                                                          k                                      +                                      1                                                                                                                                                                  +                                                                                                α                                                                                                            m                                      +                                      1                                                                        ,                                                                          k                                      -                                      1                                                                                                                                      ⁢                                                                  d                                                                                                            m                                      +                                      1                                                                        ,                                                                          k                                      -                                      1                                                                                                                                                                  +                                                                                                                                                                                                                                                            α                                                                                                            m                                      +                                      1                                                                        ,                                    k                                                                                                  ⁢                                                                  d                                                                                                            m                                      +                                      1                                                                        ,                                    k                                                                                                                              +                                                                                                α                                                                                                            m                                      +                                      1                                                                        ,                                                                          k                                      +                                      1                                                                                                                                      ⁢                                                                  d                                                                                                            m                                      +                                      1                                                                        ,                                                                          k                                      +                                      1                                                                                                                                                                                                                                                              }                                                                                                                              [                  Equation          ⁢                                          ⁢          7                ]            
However, when the data transmission signals 11 to 18 which are obtained through modulation using data unknown to the reception end, are located in the vicinity of the pilot signal 10 and transmitted, the interference component generated in the pilot signal 10 is unknown to the reception end, and the unknown interference component prevents estimation of the transmission characteristic Hm,k of the transmission channel.
Therefore, in the conventional multicarrier modulation described in Patent Document 2, a constraint condition is imposed on at least one of the data transmission signals 11 to 18 which are located in the vicinity of the pilot signal 10 and transmitted so as to suppress an interference (specific interference occurring in a quadrature phase component) in the pilot signal 10. Specifically, a constraint condition is imposed on at least one of the amplitude values dm−1,k−1, dm−1,k, dm−1,k+1, dm,k−1, dm,k+1, dm+1,k−1, dm+1,k, and dm+1,k+1. for modulating the data transmission signals 11 to 18 such that the imaginary number term in (Equation 5) indicates zero.
For example, when the constraint condition is imposed on the amplitude value dm+1,k for modulating the data transmission signal 17 to be transmitted on the k-th carrier in the (m+1)th symbol, the amplitude value dm+1,k is determined so as to satisfy (Equation 8).
                              {                                                                                                                α                                                                        m                          -                          1                                                ,                                                  k                          -                          1                                                                                      ⁢                                          d                                                                        m                          -                          1                                                ,                                                  k                          -                          1                                                                                                      +                                                            α                                                                        m                          -                          1                                                ,                        k                                                              ⁢                                          d                                                                        m                          -                          1                                                ,                        k                                                                              +                                                                                                                                                α                                                                        m                          -                          1                                                ,                                                  k                          +                          1                                                                                      ⁢                                          d                                                                        m                          -                          1                                                ,                                                  k                          +                          1                                                                                                      +                                                            α                                              m                        ,                                                  k                          -                          1                                                                                      ⁢                                          d                                              m                        ,                                                  k                          -                          1                                                                                                      +                                                                                                                                                α                                              m                        ,                                                  k                          +                          1                                                                                      ⁢                                          d                                              m                        ,                                                  k                          +                          1                                                                                                      +                                                            α                                                                        m                          +                          1                                                ,                                                  k                          -                          1                                                                                      ⁢                                          d                                                                        m                          +                          1                                                ,                                                  k                          -                          1                                                                                                      +                                                                                                                                                α                                                                        m                          +                          1                                                ,                        k                                                              ⁢                                          d                                                                        m                          +                          1                                                ,                        k                                                                              +                                                            α                                                                        m                          +                          1                                                ,                                                  k                          +                          1                                                                                      ⁢                                          d                                                                        m                          +                          1                                                ,                                                  k                          +                          1                                                                                                                                                  }                =        0                            [                  Equation          ⁢                                          ⁢          8                ]            
As described above, when suppressed is an interference (specific interference occurring in the quadrature phase component) to the pilot signal 10 from the data transmission signals 11 to 18 which are located in the vicinity of the pilot signal 10 and transmitted, an imaginary number area of the pilot signal 10 received at the reception end is known, so that the estimation of the transmission characteristic Hm,k of the transmission channel is easily performed at the reception end.    Patent Document 1: Japanese Translation of PCT International Application No. 11-510653 (Publication No.: WO96/35278)    Patent Document 2: Japanese Translation of PCT International Application No. 2004-509562 (Publication No.: WO2002/025884)    Non-Patent Document 1: S. B. Weinstein and Paul M. Ebert, “Data Transmission by Frequency-Division Multiplexing Using the Discrete Fourier Transform”, IEEE Transaction on Communications, vol. COM-19, pp. 628-634, October 1971.    Non-Patent Document 2: Burton R. Saltzberg, “Performance of an Efficient Parallel Data Transmission System”, IEEE Transaction on Communications, vol. COM-15, pp. 805-811, December 1967.    Non-Patent Document 3: R. Li and G. Stette, “Time-Limited Orthogonal Multicarrier Modulation Schemes”, IEEE Transaction on Communications, vol. 43, pp, 1269-1272, February/March/April 1995.    Non-Patent Document 4: M. A. Tzannes, M. C. Tzannes, J. Proakis and P. N. Heller, “DMT Systems, DWMT Systems and Digital Filter Banks”, IEEE International Conference on Communications, pp. 311-315, May. 1994